One concept for increasing the capacity of optical storage media is, to use holographic data storage. In this case the surface or the whole volume of the holographic storage medium is used for storing information, not just a few layers as for conventional optical storage media. Furthermore, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc.
In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator and carries the information to be recorded in the form of the data pages. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam. According to one holographic storage approach the reconstructed object beam is read in transmission (transmission type holographic storage medium). For this approach an optical system is needed on both sides of the holographic storage medium. A different approach is to read the reconstructed object beam in reflection (reflection type holographic storage medium). In this case only a single optical system is required. For this purpose the rear side of the holographic storage medium is coated with a mirror layer. The reconstructed object beam is reflected by this mirror layer and can be read from the same side as used for recording.
In both approaches a superposition of the reference beam and the reconstructed object beam may disturb read-out of the reconstructed object beam. This is especially the case when the reference beam is a spherical wave, which spreads into a large angular range. In addition, during recording the reference beam may illuminate the storage material outside the area of interference with the object beam. This effect uses up a part of the dynamics of the storage material without actually recording data.
One known solution to prevent the interference between the reference beam and the reconstructed object beam is to design the optics in such a way that the reference beam does not reach the read-out lens. This is, however, a limitation for the design. Another solution, which is disclosed for example in EP 1 080 466 B1, is to apply confocal filtering to suppress the reference beam. This, however, further complicates the optical set-up.